1. Field of the Invention
The present invention relates to an antenna device that transmits or receives radio waves with an antenna.
2. Description of the Related Art
For example, in the tire pressure monitoring system (TPMS) disclosed in Japanese Patent Application Publication (JP-A) No. 11-254925, the air pressure of tires is detected by detecting means disposed inside the tires. An electrical signal corresponding to the detection result is outputted from the detecting means, and a wireless signal corresponding to the level of the electrical signal is outputted from transmitting means.
The wireless signal outputted from the transmitting means is received by receiving means, and an electrical signal corresponding to the level of the received wireless signal is outputted from the receiving means. The electrical signal outputted from the receiving means is inputted to a control unit disposed in the vehicle. When the control unit determines that the air pressure of the tires is equal to or less than a predetermined value on the basis of the level of the inputted electrical signal, the control unit activates an alarm device and informs the passenger of the vehicle with an alarm.
The receiving means for receiving the wireless signal transmitted inside and outside the vehicle is disposed with an antenna device for substantially receiving the wireless signal, and an example thereof is disclosed in JP-A No. 10-16646. The antenna device is configured to include a rod-like antenna whose leading end side is formed in a coil and a circuit board including an electrical circuit connected to a power supply and a ground. The base end side of the antenna is electrically and mechanically connected to the electrical circuit of the circuit board.
The antenna device disclosed in JP-A No. 10-16646 is disposed inside an inner mirror attached to the inside of the vehicle cabin. In contrast, the wireless signal transmitting means is disposed outside the vehicle cabin (e.g., inside the tires) because the tire pressure monitoring system monitors the air pressure of the tires. For this reason, it is preferable to dispose a receiving antenna device outside the vehicle cabin in order to receive the wireless signal from the transmitting means. As is also suggested in JP-A No. 11-254925, the inside of a visor configuring a door mirror of the vehicle is considered as a candidate for the position where the receiving antenna device may be disposed.
When the antenna device is disposed outside the vehicle cabin, including the inside of the visor of the door mirror, the antenna and the circuit board are housed inside a case in a state where the base end side of the antenna is soldered to the circuit board in order to prevent water from adhering to the circuit board and the antenna when it rains or when the vehicle is washed, and the case is fixed to a predetermined site on the vehicle.
When the antenna and the circuit board are housed inside a case in this manner, a retaining protrusion or the like for retaining the antenna is formed inside the case and the circuit board is screwed to a predetermined site in the case in order to prevent inadvertent displacement (movement) of the antenna and the circuit board inside the case.
Here, when the antenna and the circuit board are attached to the inside of the case, error arises in the relative positional relationship between the antenna and the circuit board inside the case due to error in the position of the retaining protrusion and screwed portion inside the case, dimensional error of the antenna, and error in the positions of a hole formed in the circuit board in order to insert a screw when the circuit board is screwed to the case.
Unnecessary force (stress) acts on the antenna when such error arises because the base end side of the antenna is fixed to the circuit board with solder. When such force acts on the antenna and deformation arises in the antenna, the characteristics of the antenna fluctuate.
Further, when tension or the like acts on the solder connecting the antenna and the circuit board due to such error, cracks arise in the solder.